fix(tvix/eval): emit only warnings on shadowed outputs

Unfortunately, nixpkgs has at least one case[1] where the out environment
variable is shadowed -- though it doesn't cause a problem, since it's
shadowed with the correct value, odd as this may be!

[1]: c7c2984716/pkgs/development/python-modules/pybind11/default.nix (L19)

Change-Id: Ibf6790d2484dc9cce8e424feeb5886664d498dc3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8696
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/warnings.rs

@@ -17,6 +17,7 @@ pub enum WarningKind {
     DeadCode,
     EmptyInherit,
     EmptyLet,
+    ShadowedOutput(String),
 
     /// Tvix internal warning for features triggered by users that are
     /// not actually implemented yet, but do not cause runtime failures.
@@ -100,6 +101,11 @@ impl EvalWarning {
 
             WarningKind::EmptyLet => "this `let`-expression contains no bindings".to_string(),
 
+            WarningKind::ShadowedOutput(ref out) => format!(
+                "this derivation's environment shadows the output name {}",
+                out
+            ),
+
             WarningKind::NotImplemented(what) => {
                 format!("feature not yet implemented in tvix: {}", what)
             }
@@ -120,6 +126,7 @@ impl EvalWarning {
             WarningKind::DeadCode => "W008",
             WarningKind::EmptyInherit => "W009",
             WarningKind::EmptyLet => "W010",
+            WarningKind::ShadowedOutput(_) => "W011",
 
             WarningKind::NotImplemented(_) => "W999",
         }